A communication system comprises signalling points or nodes, such as user terminals, different exchanges, routers, switches, links, stations and so on. The communication also comprises appropriate communication media between the signalling points. Different signalling points may be situated within an element of the communication system, wherein the communication may occur with the element. The communication media may comprise, for example, a wired interface, a radio frequency interface or an optical interface. The communication may be carried by analogue or digital signals or a combination of these, such as digitally-modulated analogue signals.
Signal amplification is required in various communication applications. For example radio frequency signals transmitted between signalling points in a communication system employing radio transmission may need to be amplified during some stage of the transmission and/or reception. The signalling points may be, for example, a transmitting station and a receiving station or an intermediate node of the communication system. The amplification of the signals is required for example since the amplitude of a signal tends to be attenuated during transmission between signalling points, thereby decreasing the quality of the transmission. Also, noise becomes typically added to the signal from other sources as well as from the transmitting and receiving and the possible intermediate apparatus itself. A communication system is thus typically provided with amplifying means to compensate for the attenuation. Amplification of the signal may also be used for increasing the signal-to-noise ratio of the signal.
An input signal may be amplified by means of a power amplifier. The power amplifier may have at its input a modulated radio signal. The input radio signal may be a modulated digital signal. The power amplifier, when operated in a non-linear region, typically close to saturation produces intermodulation distortion products at its output as well as the desired carrier signals. The intermodulation distortion products are typically produced on either side of the desired carrier signals. Intermodulation distortion products are typically caused by the power amplifier not acting as a linear amplifier, which occurs when the power amplifier is operating close to saturation. The intermodulation distortion produces frequencies at multiples of the carrier frequency of the desired signals. These frequencies tend to be lower in signal strength than the desired carriers. These intermodulation distortion products increase the spectral space occupied by the signal and are therefore undesirable. One way to reduce the intermodulation distortion products is to operate the power amplifiers as linear amplifiers. The power amplifiers would be operated so that there is a substantially linear relationship between the input signal power and the output signal power.
Amplifiers that are intended to cover a range of frequencies should provide as linear performance as possible across the designated frequency band. However, an amplifier may introduce distortions. The distortion may be linear or non-linear. The linear distortions relate to bandwidth limitations. The non-linear distortions have two components called AM-AM (amplitude modulation-amplitude modulation) or AM-PM (amplitude modulation-phase modulation) distortion. In the non-linear case amplitude variations in the input signal may cause undesirable amplitude and/or phase variations in the output signal. In addition, the distortions may cause mixing between the different frequency components present in the signal. The term ‘AM-AM’ refers to amplitude dependent amplitude (gain) variations and the term ‘AM-PM’ refers to amplitude dependent phase variations.
A prior art solution for the linearity problem exploits the fact that the non-linearity increases with the output power level of the amplifier. Thus, if the input level is reduced, i.e. “backed-off”, the amplifier is arranged to operate only within its more linear region. However, this approach may not be desirable in all applications as it fails to utilise the full range of available output voltage-swing. The backing-off may have an disadvantageous effect on power efficiency of the amplifier.
The linearity of the amplifying function can also be improved by provision of a linearisation function designed to reduce the distortion. Linearisers have been developed which enable operation of an amplifier with reduced intermodulation distortion at a point on the gain/input power or phase/input power curves after which the amplifier does not act as a linear amplifier. Such a lineariser is placed in the signal path before the power amplifier and may therefore precondition the input signal before passing it to the power amplifier. In other words the gain of the lineariser increases as the power of the input signal increases and increases in such a way as to substantially oppose the typical power amplifier power characteristics. This is called gain expansion. In addition, the linearizer may change the phase of the input signal as the power of the input signal increases. The change in the phase may be such that it opposes the typical power amplifier phase characteristics. This is called phase expansion.
A lineariser should be capable of producing enough of gain and/or phase expansions to mitigate the non linear amplitude and phase problems associated with power amplifiers in a region that is close to saturation. The effect of the lineariser is to increase the effective range over which the power amplifier is linear but allows the power amplifier to operate in its more efficient non-linear range. However, the inventor has found that the prior art linearisers may not be capable of providing an optimised independent amplitude and phase tuning for linearity and efficiency of the prior art lineariser arrangements. In addition, the prior art linearisers are of substantially big size, thereby making the integration thereof with the amplifier difficult in applications requiring substantially small sized components, such as mobile phones.